In the conventional electric motor as shown in FIG. 1A, the construction is such that a stator 22 consisting of a stator core 28 and the stator coils 29 is provided, a rotor 21 consisting of a rotor core 23 and rotor coils 24 installed on a shaft 25 is provided within the stator 22, and a commutator 26 having as many poles as the number of the poles of the rotor is installed fixedly to the rotor shaft. The motor is operated by supplying an electric current through a brush 27 and the commutator 26 to the rotor coils 24 (this is the so-called straight wound motor). But in this type of motor, the electric current has to be supplied both to the stator and to the rotor, and therefore, the motor is very complicated, with the result that its volume and weight become very large, and that it is difficult to miniaturize the motor. Further, the motor is susceptible to failures, and therefore, difficulties are created in the maintenance of the motor.
Meanwhile, there is another type of conventional electric motor in which a coilless rotor 31 has a core mounted on a shaft 35 and installed within a stator 32 having ring coils 40 and a stator coil 39 as shown in FIG. 1B. This is an induction motor having a simple structure to a certain extent, but this type of motor is also difficult to miniaturize, in addition to the fact that its torque is weak, and the control of the revolution speed is impossible, thereby making its applicability very limited.